ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 387 



the embryo from the vitellus and making preparations of the isolated 

 embryo. The author treated the ova of Agelena with 10 per cent, chromic 

 acid for twenty-four hours, and effected the desirable result of isolation. 



(a) The egg envelopes. The double membrane, demonstrated with 

 acetic acid, consists, as Ludwig and Balbiani have shown, of two layers — an 

 internal vitelline membrane and a superficial chorion. In Pholcus the 

 little ovarian follicles are seen to be provided with a layer of epithelial 

 cells, probably sharing in the formation of deutoplasm. The chorion is 

 probably, however, due to the walls of the oviducts. The two envelopes 

 are quite homogeneous. Outside the chorion is a layer of refractive cor- 

 puscles, soluble in alcohol. These are for the most part due to the 

 epithelial cells of a special organ, the " uterus." (&) As to the constitution 

 of the ripe ovum the author's observations have shown him (1) that the 

 alleged division of the plasmic material into two layers does not exist, 

 although under the vitelline envelope a peripheral accumulation of proto- 

 plasm may be readily observed ; (2) that no " yolk-nucleus " is ever seen in 

 the ripe egg, and that Schiitz's interpretation is correct ; (3) that the 

 germinal vesicle does not disappear, (c) In regard to segmentation, 

 Schimkewitsch criticizes the relative observations, and notes his own. He 

 observed the division into four, eight, and sixteen segments, which from the 

 exterior looked like rosettes. The yolk-globules were disposed in columns, 

 but remained separate from one another. In sections the somewhat excen- 

 tric segmentation cavity was seen. The segments were seen as pyramids 

 internally abutting round the cavity. At the end of segmentation, a 

 section through the centre exhibits in Tegenaria and Epeira upwards of 

 forty pyramids. The contained protoplasmic mass then increases and 

 becomes polynuclear; the internal extremities are resolved into yolk- 

 spherules which fill the segmentation cavity. The protoplasmic masses 

 and their nuclei undergo remarkable modifications. The chromatin of the 

 nuclei mingles with the surrounding plasma. The protoplasmic masses 

 are transformed into blastoderm cells, which are probably separated off at 

 the peripheral extremity of the pyramids, (d) At this stage the egg thus 

 exhibits two layers — the primary ectoderm of flattened cells, and the 

 primary endoderm represented by polynuclear vitelline cells. Sometimes 

 almost simultaneously, but often successively and in different order, the 

 vitelline pyramids are destroyed, the primary ectoderm or blastoderm is 

 concentrated, and tlie mesoderm is formed. Each pyramid breaks up into 

 rounded polynuclear cells, and the destruction may occur in various direc- 

 tions. The author believes in an actual migration of blastoderm cells from 

 the dorsal to the ventral pole. But after this the dorsal side of the egg 

 reacquires a cellular mantle by the multiplication of blastoderm cells. 

 The formation of mesoderm is signalized by the appearance, on the ventral 

 surface of the egg, of a whitish spot (first stage), which increases gradually 

 (second stage) and takes the form of a tubercle (or cumulus), and before 

 this tubercle a whitish streak. The rudiment has, as Herold said, the form 

 of a comet. Fourthly, before the cumulus primitivus a white spot appears, 

 at first united to the cumulus by the streak above mentioned, but in the 

 fifth stage separated from it by a depression. In regard to the first mesoderm 

 cells, Schimkewitsch believes that in those forms where the concentration 

 of the blastoderm precedes the formation of the mesoderm, the destruction 

 of the vitelline pyramids occurs equally throughout the egg, and the first 

 mesodermic cells are separated from the internal vitelline cells. In those 

 forms where the mesoderm is formed before the concentration of the blas- 

 toderm, the destruction of pyramids takes place more energetically on the 

 ventral surface, and the first cells are separated from two pyramids placed 



